The braking systems for vehicle wheels comprise braking actuators (which may be hydraulic or mechanical) for applying a force on friction elements for the purpose of generating a braking torque tending to slow the vehicle down.
Most braking control systems known in aviation make use of a setpoint that is converted either into a pressure for hydraulic brakes, or, for brakes having electromechanical actuators, into a force to be applied by the pusher or into a movement to be performed by the pusher.
With electromechanical brakes, controlling actuators in position raises a particular problem associated with possible expansion of the components of the brake during braking. During high-intensity braking, e.g. braking a fully loaded airplane after aborting takeoff, the heat given off by the friction elements of the brake is very large and runs the risk of causing the torsion tube on which the friction elements are mounted to expand, which can have the effect that of reducing the level of force that is exerted for a given position of the pusher of the actuator.
In particular, when using a brake with carbon friction elements, the expansion of the friction elements themselves is very small compared with the expansion of the metal torsion tube on which the friction elements are placed, which can lead, in the event of a very large amount of heat being given off, to a drop in the braking force for a given position of the pushers of the actuators.